The present invention relates generally to a method for applying thermal barrier coatings to metal pieces and the resulting pieces, and more particularly to a method for applying a coating system to a gas turbine engine combustion chamber liner having cooling holes and the resulting liner.
Various methods are used to protect metal pieces exposed to high temperature environments. For instance, cooling air is sometimes blown over the piece. In some applications such as aircraft engine combustion chamber liners, cooling holes are formed in the liner for directing film cooling air through the liner and over surfaces of the liner exposed to high temperatures. The film cooling air cools the liner and forms a fluid barrier between the liner and hot gases which flow through the engine to prevent the gases from directly contacting the liner.
In addition, thermal barrier coating systems are applied to surfaces of metal pieces exposed to high temperature environments to reduce the amount of heat transferred to the piece. However, applying thermal barrier coating systems to pieces having cooling holes may cause the holes to become blocked thereby reducing cooling. In order to overcome this problem, the cooling holes in new pieces are often formed (e.g., by laser drilling) after the piece is coated. However, forming the cooling holes after the piece is coated generates significant heat which can negatively affect the life of the piece. To avoid this problem, the cooling holes are sometimes made first and masked before applying the coating to ensure the holes are not blocked by the coating. However, masking increases the manufacturing cost. Methods of removing coatings from the cooling holes using high pressure fluid jets have been developed to eliminate the need for masking. Although these methods work well for metal pieces having cooling holes which are angled in a uniform direction, a method for coating pieces having film cooling holes angled in more than one direction has not been developed.
Briefly, the present invention includes a method of applying a thermal barrier coating system to a metal piece having a first plurality of cooling holes angled in a first direction through the piece from a first surface of the piece to a second surface of the piece opposite the first surface, and a second plurality of cooling holes angled in a second direction different than the first direction through the piece from the first surface to the second surface. The method comprises spraying a bond coat on the first surface of the piece at angles with respect to the first direction and the second direction and to a thickness selected in combination with the angles to prevent the bond coat from entirely filling any hole within the first plurality of cooling holes or any hole within the second plurality of cooling holes. Further, the method includes spraying a thermal barrier coating on the bond coat at angles with respect to the first direction and the second direction and to a thickness selected in combination with the angles to prevent the thermal barrier coating from entirely filling any hole within the first plurality of cooling holes or any hole within the second plurality of cooling holes. A high pressure fluid jet is sprayed from a nozzle assembly through each hole within the first plurality of cooling holes in a direction generally parallel to the first direction and through each hole within the second plurality of cooling holes in a direction generally parallel to the second direction.
In another aspect, the invention includes an annular liner for use in a combustor. The liner comprises an annular shell surrounding an axial centerline having an upstream end, a downstream end, a first plurality of cooling holes angled in a first direction through the shell from an outer surface to an inner surface, and a second plurality of cooling holes angled in a second direction different than the first direction through the shell from the outer surface to the inner surface. Further, the liner includes a thermal barrier coating system applied to the inner surface of the shell. Fluid flow through each of the holes in the first plurality of cooling holes and the second plurality of cooling holes is substantially unobstructed by the thermal barrier coating system.
Other features of the present invention will be in part apparent and in part pointed out hereinafter.